1. Field of the Invention
The present invention pertains a method of determining elastic and plastic mechanical properties of ceramics, intermetallics, and other hard, brittle materials which fracture prior to plastically deforming when tensile loads are applied. The invention provides an improved means to characterize the plastic and elastic deformation of these materials. It is particularly suitable as a materials characterization method and quality control method for manufacturers and users of these materials. The method measures the plastic and elastic deformation of hard, brittle materials with low ratios of elastic modulus to hardness.
2. Description of the Prior Art
In order to comprehend the mechanisms of erosion and abrasion of ceramics, it is important to understand the load displacement relationship between an eroding or abrading particle and the material being eroded or abraded. An important part of the method to ascertain this load-displacement relationship is the determination of the stress-strain curve of the material.
In the past, the mechanical properties of hard, brittle materials have been measured by a uniaxial compression test. However, these materials fracture prior to plastically deforming in a uniaxial compression test. Uniaxial compression tests are described in P. G. Meredith, "Fracture and failure of brittle polycrystals: an overviews in Deformation Processes in Minerals, Ceramics and Rocks, edited by D. J. Barber and P. G. Meredith, Unwin Hyman Ltd., 1990, pp. 5-47; and S. D. Hallam and M. F. Ashby, "Compressive brittle fracture and the construction of multi-axial failure maps" in Deformation Processes in Minerals, Ceramics and Rocks, edited by D. J. Barber and P. G. Meredith, Unwin Hyman Ltd., 1990, pp. 84-108; among others. Mechanical properties of hard, brittle materials can also be measured by compression tests with confining hydrostatic pressure. The confining pressure can be applied through a gas, liquid or a solid medium. These apparatus are difficult to operate and are time consuming to use. When a gas or liquid is used as a confining medium, only low pressures can be applied to the test materials. Because of friction of seals and the strength of a solid medium, tests with confining pressures can be prone to considerable errors. Compression tests with confining pressures are also described in the above references. The advantages of the invention over the prior art are that the invention is easier to use and is less prone to error than the prior art. Indentation techniques are known in the art. These include J. S. Field and M. V. Swain, "A Simple Predictive Model for Spherical Indentation," J. Mater. Res., 8 [2] 297-306 (1993). However, the analyses of the contact mechanics used for these prior models do not adequately describe the load-displacement relationship between an eroding particle and the surface of the brittle material because the analyses do not adequately take into account the strain hardening of the brittle material as well as the elastic deformation. This invention characterizes such brittle materials by measuring the result of the indentation of a spherical indenter (a sphere or spherically tipped cone) and determines both plastic and elastic deformations resulting. This invention improves on other analytical indentation processes and takes into account the strain hardening of hard, brittle materials as well as the elastic deformation.